Low coolant light & bad idle
 

I'm noticing this really strange gremlin after I've parked my car on a declining stretch of road (nose down):
upon starting the engine, the 'low coolant level light' stays on, and the engine idles very rough, as if running on three cylinders (out of four). After 20 seconds the idle runs fine again, and the coolant light goes out after driving some 50 yards. Coolant level is fine. Sometimes the rough idle also happens on starting without the coolant light on.

What could be wrong here??

I don't know very much, but I'll try to help.

I'm not sure about the coolant light. I expect there's a sensor issue or something. I get the coolant light thing on my 190, but it's always because it's using too much coolant!

As for the rough idle, I had the same thing and first time it was the throttle control valve (around $300, but possible to clean. it gets sticky and gunky, which can make your problem intermittent. sometimes you can salvage what you've got by cleaning it). One of the super-mechs on here will be able to tell you where it is and how to clean it for your model.

The most recent time it was rough I had to do my plug wires. Turns out one of them was cut so I would drop a cylinder or have a rough one from time to time (particularly when the weather was bad).

There are probably a dozen other possibilities (something in the fuel system - pump, filter, etc.), but I don't know enough to make any other suggestions.

Hope this helps!

Thanks Russ,
I indeed suspect the coolant level sensor for the faulty light, maybe a corroded connector or something.
The strange thing about the bad idle is the fact that it always completely dissappears after some 20 seconds. Makes me think it's something in the start-up programming in the electronics, instead of plugs, injectors etc. that would give the problem all the time, I'd say.
Went to MB recently, but -yep- problem didn't occur then :mad:. They had a look at the throttle valve, did some adjusting but basically no problem there.

Turned out to be a leaking headgasket, hardly ever happens on this engine. Shop pressurized the cooling system, and inspected the cilinders with some sort of endoscopic camera :eek: where they discovered coolant entering. Level signal was caused by a faulty level sensor in the screen fluid reservoir.

Now the auto box and diff are starting to whine...